Phonograph pickup



p 1945- A G. SMITH 2,383,994

. PHONOGRAPH PICK-UP- Filed May 25, 1942 Patented Sept. 4, 1945 PHONOGBAPH PICKUP Graydon Smith, Concord, Mass., assignor to National Company, Inc., Malden, Mass, a corporation of Massachusetts Application May 25, 1942, Serial No. 444,323

11 Claims. (Cl. 179-10041) This invention relates to the art of reproducing sounds from phonograph records and consists in a new and useful pick-up. More specifically, my present invention comprises improvements in an electro-dynamic pick-up of the type shown in my prior Patent No. 1,915,804, dated June 27, 1933.

The most important object of my invention is to produce an electro-dynamic pick-up in which the output voltage is a faithful counterpart of the signal impressed on the record. In my earlier pick-up the upper limit of frequency reproduced approached 5,000 C. P. S. and there were prominent resonance peaks which reduced the efliciency of the unit, whereas my present instrument herein disclosed reproduces at a frequency of 20,000 C. P. S. which represents a very substantial gain and greatly increased efliciency.

Another object of the invention is to produce a pick-up characterized by extremely light contact pressure of the stylus or needle on the record in order to increase the life of the'record.

An important feature of the invention resides in a novel armature incorporating a permanently mounted stylus, so constructed that the working portion of the armature is located closely adjacent to the stylus.

A further feature of the invention is the division of the secondary coil into two portions disposed about opposite legs of a core member and so connected that spurious voltage introduced by the presence of stray magnetic fields is reduoed and the signal correspondingly more faithfully reproduced.

These and other objects and features of the invention will be more readily understood and appreciated from the following detailed description of a preferred embodiment thereof selected for purposes of illustration and shown in the accompanying drawing in which- Fig. l is a plan view of a completed pick-up, the cover being removed to expose the interior construction,

Fig. 2.is a view in cross section along the line 2-2 of Fig. 1, showing the cover in position,

Fig. 3 is a view in cross section along the line 3-3 of Fig. 1,

Fig. 4 is a view in perspective of the combined armature and stylus, and

Fig. 5' is a diagrammatic view showing the the groove of a phonograph record, a voltage is induced in the armature by reason of the fact that a portion thereof is oscillated through the magnetic field. Since the armature forms a closed primary circuit about the core of a transformer, a current is induced in the secondary and the latter is connected to the input end of a suitable amplifier.

In the construction which I have developed, the pickeup is housed in a case 10 which may conveniently be formed as a casting but which may be molded from any suitable non-magnetic material. The casting includes an elongated rectangular shank piece I2 which maybe tapped of magnetic steel is which is spot welded or otherneedle holder and armature disclosed in my priorpatent No. 1,915,804.

The principle upon which my improved pick-up" operates is the same as that disclosed in my prior patent above referred to, that is to say, an armature in the form of a closed loop linking the core off at an angle of approximately 45.

wise secured to a plate 14 also of magnetic material, which lies along the bottom of the case to and is tapped to receive a screw H. An L-shaped permanent magnet 18 is secured to the plate It by means of a screw IS, the bottom of the magnet is being machined to fit tightly against the plate is and the other end'of the magnet being ground The magnet |8 may comprise any of the usual alloys or types of steel used in the manufacture of permanent magnets. Integrally formed in the bottom of the case it is a pair of angularly disposed lugs 20 which are tapped at an angle to receive a pair of screws 26. A core comprises a stack of L-shaped steel plates 22 bound together by the screws 24 and by two other screws 26, the screws 26 being employed to mount the stack in the bottom of the, case [0. The plates 22 may be formed from silicon steel or from any suitable magnetic material having relatively low magnetic reluctance, and the stack is formed as a hollow square. Around each side members of the core 22 is disposed a coil 28, the coils being connected in series and connected to the leads H.

In the forward lower corner of the case Hi there is an integral shelf-like projection 30 provided with a V-shaped channel 3! above which is mounted a cap piece 32 having a v shap'ed recess forme din its central portion and secured to the shelf 30 by means of screws 34. On the opposite side of the core 22 there is another projection 38 which is a part of the bottom of the casting forming the case I 0. The projection 38 is also provided with a V-shaped channel aligned with the channel in the shelf 30, A cap piece 36 having a V-shaped recess formed in its central portion is secured to the projection 38 by means of a pair of screws 40. The cap pieces 32 and 36 together with their channeled projections 30 and 80 form bearings for an armature 42 shown in Fig. 4.

The armature l! is the most important element of the pick-up. The essential attribute is its light weight, and I have folmd that magneslum or aluminum are most suitable, the former being preferable. The armature 42 is shaped as a hollow triangle having an upper arched or crowned leg 44 and a pair of straight legs leading to a cylindrical socket in which is secured a stylus 4| having a polished point of diamond. sapphire, or other suitable material. Near the upper corners of the armature 42 is a pair of projecting studs ll which are of diamond-shaped cross section and upon which are mounted relatively thin sleeves I! of resilient rubber. In assembling the pick-hp, the L-smped plates 22 are passed through the hollow triangle of the armature 42 with the result that the armature is looped about the lower leg of the core 22. The plates in the lower leg may be g und or flied toaccommodate the armature freely. The studs II are inserted in the bearings formed by the cap pieces 31 and 38, the rubber sleeves i2 providing .a resilient mounting for the armature l2. Therubber sleeves II also serve to damp the armature and to keep it properly centered. It

will be seen from an inspection of Figs. 1 audit that .the forward end for angular face of the permanent magnet ll fits nicely against the unders'ide of the upper left of the square formed by the plates 22 so that the latter forms one poleof' the, permanent magnet. The parts are so dimensionedthat there is a space between the forward end of the plate It and the lower-leg of the core 22. Through this gapthe side 48 of the armature If is disposed, the end of the plate ll being ground of! so that it is parallel with the leg ll of the armature l2 and with the underside of the lower-leg of the core 22.

The pickup is provided with a cover 54 having downwardly projecting lugs 58 which are internally tapped to receive a pair of screws 58 which serve to hold the cover in place.

- It will now be .evident that when the stylus is placed upon a moving phonograph record the configuration of the traveling groove in the record will cause the stylus Il to oscillate in accordance with the nature ofthe groove. The lower portion of the side ll of the armature l2 isc'ausedtooscillateinthegapbetweenths end of the-plate I. and the lower leg of the core 22. ..8in'ce'the plates II and ii are of magnetic material and since the core 22 is also magnetic, there is a magnetic circuit through the permanent magnet, the stack of plates 22, the air gap,

and the plates l8 and it. When the side it of the armature 42 oscillates in the air gap, a current of electricity is caused to fiow through the whole triangle of the armature, in accordance with thewell understood principle that a volta e is induced in a conductor when it is moved through a magnetic field. The armature 2 thus acts as the primary winding of a transformer in which the stack of plates 22 forms the core and the coils ll, the'secondary circuit. It will be observed'that the rubber sleeves I2 serve to insulate the armature from the case.

While the pick-up of my invention will operate if the secondary coil is wound anywhere upon the" core 1!, the provision of two coils connected in series is preferable because the coils will tend to cancel induced voltages which may be present from stray m netic fields coming from outside sources, for example, from a so cycle motor used to'drive the turntable supporting the record heneath the pick-up. Although the use of opposed coils to buck hum is well understood, I am not aware that such an arrangement of coils has- No. 1,915,804 I stated that the armature could i be formed from metal such as silver or copper.

- These metals are of course extremely good conductors of electricity. However, for an armature of given weight and perimeter the lightest material will provide the thickest cross section and thus display the greatest rigidity, other.

things being equal. The resultant rigidity makes the natural resonance frequency of the armature exceedingly high. Inpractice, I have found it desirable to place the conductivity-to-weight criterion second to the stiffness criterion, and on I this basis magnesium or-aluminum are preferable .to silver or copper. The relatively great rigidity thus obtainedresults in an armature which will respond uniformly over a very great range of frequencies. The requirement that the armature be a good conductor of electricity can be met by the exercise of care in design, that is to say, by giving the armature a more extensive cross section and a shorter perimeter in order to lower the resistance thereof.

In Fig. 5 I have reproduced diagrammatically the armature disclosed in my prior Patent No. 1,915,804. The armature was in the shape of a relatively large rectangle having a socket integral with one corner thereof for the reception of a phonograph needle M. The upper leg of the rectangle was disposed between pole pieces 68 and the lower leg was disposed below a piecelll having a triangular cross section and pro-' viding a pivot point for the armature. A phonograph record- It is shown as in place below the needle 64. when the needle 64 was vibrated. the entire armature was also vibrated, pivoting aboutthe piece 10. It will be apparent that a relatively long beam was present, comprising the length of the needle and the side 62 of the armature. Experiments have informed me that the vibration of the upper end of the long beam was not a faithful counterpart of the vibrations of the needle, because the upper end "whipped much as the tip of a fishing rod will have a movement which does not correspond with the movement imparted to the butt. Furthermore, the armature. being unsymmetrical, there was' also a torsional whip or weaving. The ability of the upper end of the armature to vibrate independently of the needle results in resonances which cause poor frequency response and which increase record wear by introducing a reactive component at the needle. The lack of rigidity also has an adverse. effect on the ability to reproduce transients.

The armature 42 shown in Fig. 4 is a distinct improvement for several reasons. The triangular construction provides greater rigidity-and it will be noticed that the working con'ductorilland the needle are on the same side of the pivot no relatively free corner corresponding to the upper right hand corner of the armature as.

studs 50. Furthermore the working portion 4! and the needle 48 are relatively much closer together than thecorresponding elements of the armature shown in Fig, 5. Furthermore there. is

shown in Fig. 5. It is important to note that the armature is symmetrical and that all inactive metal has been eliminated from it. Finally, the armature 42 may be conveniently made much smaller and lighter than the type of armature shown in Fig. and for that reason, and because the leg 44 is located substantially in the axis of vibration, the moment of inertia of the armature is low. In my improved construction there is a relatively short stiff section of the armature between one pivot point and the needle and passing through the air gap.

A further improvement consists in the use of the stack of plates 22 as a portion of the magnetic circuit including also the magnet l8 and .ingly, in construing the appended claims it should be borne in mind that the important feature closely adjacent the needle is disposed within the gap of the magnet.

5. A device for reproducing sound, which comprises, a non-magnetic casing, a stack of steel plates formed as a hollow square and secured to said casing, a steel plate set inthe bottom of the casing and having one end separated by a short gap from said stack of steel plates, an L-shaped permanent magnet having one end disposed on the steel plate set within the casing and its other end in contact with the upper portionof the stack of plates, a triangular armature linked through said stackof plates and having one corner projecting below the casing, bearings in the casing supporting the upper corners of the triangular armature and causing a'portionof one leg of the armature adjacent tures of the armature are the close juxtaposition of the working portion of the conductor and the needle, the light weight of the material used and the rigidity thereof. It is also contemplated that the plates 22 may be formed as a ring or other shape. Such changes in form but not in function are within the scope of the invention as defined in the appended claims.

Having thus described my invention what I claim as new and desire to secure by Letters Patent: v

1. An electro-dynamic phonograph pick-up, which comprises a magnet shaped to present an air gap between its poles, a substantially triangular armature having a portion adjacent one corner disposed in the air gap between the said poles, a secondary coil inductively coupled to said armature, and a stylus secured to said armature at the corner thereof nearest the portion lying between the poles of the magnet.

2. A device for reproducing sound, which comprises,. a magnet shaped to present an airgap between its poles and including a hollow steel member forming one of its poles, a hollow armature linked through said hollow steel member, and having a portion thereofdisposed in said air gap, said armature including a relatively short rigid section extending from said air gap, and a stylus secured to the armature at one end of said short rigid section.

3. A device for I reproducing sound, which comprises a looped magnet having a gap between its poles and including a hollow steel member forming one pole of the magnet; a pivotally mounted light metal armature linked through said hollow steel member, and a. stylus secured to said armature, -the section of the armature between the pivot and the stylus and passing through the gap being relatively short and rigid.

4..A device for reproducing sound, which comprises a looped magnet having a gap between its poles, a hollow core forming one of the poles of the magnet, a substantially triangular armature looped through said hollow square. bearings supporting the two upper corners of the armature, and a stylus mounted at the third corner of the armature, the armature and core being so assembled that a. portion or the armathe lower corner thereof to remain in the gap, and a stylus secured to the lower corner of the armature.

6. A device for reproducing sound, which comprises a magnet shaped to present an air gap. a hollow member of magnetic material formin one pole of the magnet, an armature linked through said hollow member and having a portion thereof disposed in said gap, a pair of coils wound about opposite sides of said hollow member and connected in series, and a stylus secured to said armature adjacent said air gap.

'7. In a device for reproducing sound, a symmetrical armature of light metal having the shape of a triangle, a socket for a stylus formed in one apex of the armature, and a pair of opposed lugs disposed adjacent the other two apices thereof and adapted to serve as pivot mountings for the armature located substantially in line with one of the sides of the armature.

8. In a device for reproducing sound, a hollow triangular armature of light metal having a the armature.

10. In a device for reproducing sound, a ferrous member providing a magnetic circuit including an air gap, an armature comprising a closed frame mounted for vibration. linked through said magnetic circuit and having a portion thereof disposed in said air gap, and a stylus secured to said armature adjacent said air gap, that portion of the armature lying between the stylus and the air, gap being rigid, relatively short, and-substantially straight and located on the same side of the axis of the armature movement as said stylus.

11. In a device for reproducing sound; a magvnetic body providing a circuit including an air gap. a magnesium armature comprising a closed metal frame mounted for vibration, linked through said circuit and having a portion thereof disposed in said air gap, and a stylus rigi ly secured to the armature adjacent the air gap on the same side of the axis of armature movement :srthe-portion thereof which is located in the 8 p Gannon sm'rn. 

